Method and apparatus for refining a molten material

ABSTRACT

A method for the directional solidification of silicon or other materials. A cooled plate is lowered into a silicon melt and an ingot of solid silicon is solidified downwards.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of U.S. patent application Ser. No. 11/916,898,filed Dec. 7, 2007, which was a 371 of PCT/NO2006/000174 filed May 10,2006, which claimed the priority of NO 2005/2832 filed Jun. 10, 2005,all three Applications are incorporated herein by reference.

FIELD OF INVENTION

The present invention is concerned with refining or purifying a materialwhich can be melted and solidified. It is particularly, but notexclusively, applicable to the purification of metals, especiallysilicon, for example refining of silicon feedstock for the manufactureof solar cells.

BACKGROUND ART

Directional solidification is widely used in the Photo Voltaic [PV]industry to produce ingots, which are sliced into wafers and laterprocessed to become solar cells. The current state of the art isdominated by a system where silicon is solidified directionally from thebottom to the top in a crucible of quartz.

The same principle can be used to refine silicon to produce feedstockfor the PV industry. The directional solidification can then be used tocontrol the impurities by segregation, both the absolute levels andrelative levels of different elements (Al, Ca, Fe, Ti, Mn, B, P, etc.)can be adjusted. In addition, the process has to take into account theparticles formed in the process and particles from the incoming silicon.

A disadvantage of today's approach is that the quartz crucible can beused only once, since it is destroyed due to a phase transition of thecrucible material during cool down of the silicon ingot (and thecrucible). In addition, an anti-sticking layer of, for example,Si.sub.3N.sub.4 is required for the quartz crucible approach, in orderto avoid adhesion of the silicon.

It is an object of the invention to provide an improved solidificationprocess which results in a reduced contamination of the ingot cast.

It is a further object of the invention to provide a system which canrefine a molten material, such as silicon, without the need to replacethe container for the molten material between the casting of each ingot.

DESCRIPTION OF THE INVENTION

According to one aspect of the invention, there is provided a method ofrefining a material which comprises the steps of: forming a melt of thematerial in a vessel; bringing a cooled surface into contact with thesurface of the melt, allowing the molten material to solidify and adhereto the cooled surface; and progressively solidifying the molten materialdownwards to form a solid ingot of the material adhering to the cooledsurface.

Although defined as a method of refining, the invention could also beconsidered to be a method of directional solidification.

Thus, the invention provides a streamlined production process in whichthe furnace vessel is heated and the ingot cast, but there is no ingotcontact within the vessel and so the ingot can be removed and the vesselrefilled. The vessel does not need to be cooled between ingot castings.

Preferably, the walls and bottom of the vessel are heated. Preferably,the melt is maintained in an inert or controlled atmosphere. The methodis particularly suitable for the refinement and purification of metals,such as silicon.

The method has the advantage that the impurity level in the ingotdecreases relative to the remainder of the melt, as the ingot forms.Then the ingot is removed from the vessel or crucible and the remainingliquid with high impurity content is poured off and possiblyreprocessed. The vessel does not have to be destroyed and can bere-used. The nucleation site is simplified from a crucible to a surfaceof a plate or several plates arranged side by side. The plate or theplates arranged side by side consist of several plates in a layeredstructure. The cooled surface may be formed with discontinuities to helpensure adhesion of the ingot.

Thus, by adopting the invention, the refining process is optimized inseveral ways.

Segregation is used to refine and control the metallic impurities. Theimpurities will be pushed from the interface between solid and liquidsilicon and into the bulk liquid. A required resistivity of the refinedmaterial can be obtained by segregation and doping (before or during thecasting). The absolute resistivity level will be determined by end-userprocess and requirements.

Particles with higher density than molten silicon are removed. Theparticles that are brought into or formed during the directionalsolidification will settle to the bottom if they have a density that issufficiently higher than molten silicon. They can form a dense layer atthe bottom of the molten bath.

Particles with a density, lower or little higher, than silicon can alsobe removed. They will be pushed in front of the interface between thesolid and molten silicon. These particles will follow the convectiveflow pattern in the vessel if they are pushed sufficiently out into thebulk liquid.

The solidification process may be optimized by combining forces thatmove the impurities with flow patterns and settling forces. Moltensilicon with a high content of impurities with high density will flowfrom the solidification interface towards the bottom. A similar picturewill occur for the heavy particles, while particles with little or nodifference in density will follow the flow in the vessel. Thesemechanisms can be optimized more easily if solidification takes placefrom the top of a bath to bottom.

The directional solidification from top to bottom is therefore betterable to control the impurities than by growing from the bottom of amolten bath. The solidification can be carried out until a givenfraction is solidified (a given ingot height or size is obtained). Theremaining silicon liquid will contain a higher proportion of impuritiesand heavier particles than the starting material and can be transferredfrom the container by pouring etc. The main particulate impurities tendto be SiC, Si.sub.x,N.sub.y or Si.sub.x,O.sub.yN.sub.z.

According to another aspect of the invention, there is providedapparatus for refining a material, which comprises a vessel having abottom and sidewalls arranged to house the material in a molten stateand a cooled plate which is movable into and out of the top of thevessel.

Preferably, the vessel has heated walls and/or bottom. Preferably, thewalls and/or bottom of the vessel are made from a heat conductive butchemically inert and temperature resistant materials such as graphite,silicon nitride, silicon carbide, silica, alumina, silicon oxynitride orother ceramic oxides.

Preferably, the cooling plate includes a plurality of layers including aheat conductive layer in operative contact with cooling means, and acontact layer for making contact with the molten material. Preferably,the contact layer and any intermediate layer are made from heatconductive but chemically inert and temperature resistant materials suchas graphite, silicon nitride, silicon carbide, silica, siliconoxynitride, alumina or other ceramic oxides. The heat conductive layermay be of a metal, such as copper, aluminium or some suitable alloy. Inaddition, a heating layer such as a layer of electric resistance heatingelements or an induction heated layer may be incorporated in between theother layers. This may provide better control of the cooling process.

The plate may include an intermediate layer which is attached, to theconductive layer and which may form a sliding or snap fit with thecontact plate. There is preferably a gas-tight cover over the vessel tomaintain the inert or controlled atmosphere.

SHORT DESCRIPTION OF THE DRAWINGS

The invention may be carried into practice in various ways and someembodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a vertical section through an apparatus for carrying out theinvention;

FIG. 2 is a section on the line AA in FIG. 1;

FIG. 3 is a schematic section showing possible corrective flow patternsin the molten material;

FIG. 4 is a schematic section through a temperature controlled plateaccording to one embodiment;

FIGS. 5 and 6 are views similar to FIG. 4, showing two alternativeembodiments.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show an apparatus for the directional solidification ofmolten silica. The apparatus comprises a vessel 11 with a heated bottom12, heated end walls 13 and heated side walls 14. The vessel 11 is linedwith an outer lining 15 and an inner lining 16. The material for thoselinings should be heat conductive but chemically inert and temperatureresistant, and suitable materials include graphite, silicon carbide,silicon nitride, silica, alumina, silicon oxynitride or other ceramicoxides. The inner lining 16 defines an enclosure 17 for a silicon melt18.

Above the enclosure 17 and melt 18, there is a temperature controlledplate 19. The plate 19 is suspended from supports 21 and comprises aconductive layer 22, an intermediate insulating layer 23 and a contactlayer 24. The conductive layer 22 has a series of cooling pipes 25 andthe contact layer 24 has a roughened contact surface 26.

The conductive layer 22 is typically made from a conductive metal suchas copper or aluminium. The cooling medium in the pipes 25 is anysuitable liquid/gas, such as water or oil. The insulating layer 23 andcontact layer 24 are made from a heat conductive but temperatureresistant and chemically inert material, such as graphite, siliconcarbide or silicon nitride.

The vessel 11 and plate 19 are covered by a gas-tight cover 30. Thishouses an inert atmosphere 27 above the melt 18. There are also twoinsulated doors 28 which can be deployed over the enclosure 17 if thecover 30 is removed.

In use, silicon is placed in the enclosure 17 and heated by means of theheated bottom 12 and walls 13, 14 until it forms a melt 18.Alternatively, molten silicon can be charged directly into the enclosure17. The plate 19 is lowered so that contact surface 26 is slightlysubmerged beneath the surface of the melt 18. The cooling effect appliedto the plate 19 from the pipes 25 causes the silicon melt 18 to solidifyand adhere to the contact surface 26, forming an ingot 29 of solidsilicon.

The plate 19 is then raised so that it is above the level of the melt 18but the ingot 29 is still submerged. Further cooling then causes more ofthe silicon melt 18 to solidify, with the result that the ingot 29 growsdownwards.

When the ingot 29 has reached the required size, it is raised clear ofthe enclosure 17 and removed. The enclosure 17 is re-charged withsilicon and the insulated doors 28 are deployed over the enclosure tomaintain the molten state of the silicon. Meanwhile, the silicon ingot29 and contact layer 24 are removed from the plate 19 for use in furthermanufacture, either to ingots for remelting to produce ingots for themanufacture of wafers for solar cells or for the direct manufacture ofwafer for solar cells. The contact layer 24 is replaced and process isrepeated.

It will be understood that impurities heavier than molten silicon willfall away from the ingot 29 in the melt 18, while lighter contaminantswill circulate within the melt, due to convection forces. In this way,the impurities and contaminants present in the melt 18 will tend toremain in the melt 18 and will not be captured within the ingot 29. Thiswill have the effect of purifying the silicon forming the ingot 29. Itwill also, in turn, have the effect of concentrating the impurities andcontaminants in the remaining melt 18. For that reason, this moltensilicon remaining after withdrawal of the ingot 29 may be removed andreplaced with fresh silicon.

The direction of circulation of the molten silicon in the melt 18 isshown in FIG. 3. The arrows show the movement due to convective forces.The liquid silicon at the heated walls 13,14 is less dense and flowsupwards 31. It cools on contact with the ingot 29 and flows down 32 inthe middle of the melt 18, where it is furthest away from the source ofheat. The denser impurities tend to fall 33 and form eddies 34 at thebottom 12 near the walls 13,14.

FIG. 4 shows a preferred form for the plate 19. The conductive layer 22is formed of copper and includes cooling pipes 25. The intermediatelayer 23 is formed with an undercut profile 41 along each longitudinaledge. The contact layer 24 is formed with a corresponding overhangprofile 42 along each longitudinal edge. The contact layer 24 is simplyslid into the intermediate layer 23, with the overhang profiles 42supported on the undercut profiles 41.

It will be understood that this arrangement provides little conductiveheat transfer between the two plates, since contact is along tworelatively narrow lines. It may therefore provide a slow cooling effecton the ingot, providing time for the required crystal orientation in theingot as the material solidifies.

FIG. 5 shows an alternative arrangement in which cooling tubes 51 arelocated between an upper layer 52 and intermediate layer 53. Both layers52,53 may be formed of graphite, silicon carbide, silicon nitride, orthe like.

FIG. 6 shows an alternative arrangement which includes a heated layer 61located between two intermediate layers 62 and 63. A conductive layer 64with cooling pipes. 63 are located above the intermediate layers 62,63.This arrangement may provide an improved temperature control and alsoallows the contact layer (not shown) to be raised to a temperature abovethe melting point of silicon prior to immersion. This avoids undesiredchilling of the silicon melt in the initial stages of solidification.

It will be understood that while the preferred embodiments have beendescribed with reference to silicon, the invention is applicable to thedirectional solidification (and refinement) of other materials.

1. A method of refining a material, which comprises the steps of:forming a melt of the material in a vessel; bringing a temperaturecontrolled contact surface into contact with the surface of the meltsaid temperature controlled contact surface having no contact with thevessel sidewalls; allowing the molten material to solidify and adhere tothe temperature controlled contact surface; and progressivelysolidifying the molten material downwards to form a solid ingot of thematerial adhering to the contact surface.
 2. The method of claim 1,wherein the temperature controlled contact surface is cooled.
 3. Themethod of claim 1, wherein the walls and bottom of the vessel areheated.
 4. The method of claim 1, wherein the melt is maintained in aninert or controlled atmosphere.
 5. The method of claim 1, wherein theliquid material remaining after solidification of the ingot contains ahigher level of impurities than the starting material and is removedfrom the vessel.
 6. The method of claim 1, wherein the material issilicon.
 7. The method of claim 1, wherein the silicon is doped toprovide the required resistivity in the final solidified ingot.